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Slender Man, or Slenderman, is a very modern nightmare. A fictional bogeyman, born in the netherlands of the internet, he inspired stories and legends that originally proliferated in much the same way as "fake news" or any other compelling online meme. People saw him, thought he was scary...and decided he needed to be shared.

In 2014, however, the lines between fictional horror and real-world tragedy were blurred after two Wisconsin girls, both aged 12, cited the character as the inspiration behind the violent attempted murder of a third girl, also 12.

While the victim, thankfully, survived, the actions of the two friends, Morgan Geyser and Anissa Weier, sparked widespread debate about the Slenderman character and its influence, and reignited conversation surrounding children and "dangerous" media that recalled the era of the so-called video nasties, and anxieties about whether horror movies could trigger violent behaviour. In 1993, for example, the Child's Play series (in particular Child's Play 3) were linked, in both court and the national press, to the murder of toddler Jamie Bulger.

The Wisconsin trial is still ongoing but a documentary about the case, Beware the Slenderman, will be broadcast for the first time on SoHo Wednesday 25.

Made by Irene Taylor Brodsky, it features interviews with the families of the two defendants, and explores why they became so fixated upon the online character.

Appearance-wise, Slenderman is often depicted as a man in a suit, lurking menacingly in the background of a photograph. He's a figure you sometimes have to search for, before you clock (with an involuntary shudder) his inhuman height, elongated, unsettlingly thin figure...and the fact that he has no face.

Like many others, I first encountered the character in the online series Marble Hornets, which began in 2009 and presents itself as found footage. In the videos, created by Troy Wagner and Joseph DeLage, a young man (played by Wagner) investigates disturbing events relating to his friend Alex (DeLage), and Alex's abandoned movie project, also titled Marble Hornets.

The very first appearance of the monster, however, can be traced to a few weeks earlier. Slenderman was originally created by Eric Knudsen, in response to a message board thread in which users were encouraged to edit existing photos to make them frightening.

Knudsen's authentically chilling doctored images (one of which is shown below, with the captions and fictional context he created to go with it) caught the imagination of readers, who began making their own Slenderman images and artworks.

Creepypastas related to the character soon abounded - the word, for any non-millennials reading, refers to a sort of viral online horror story, whose chief selling point is that it is presented as "true" - and Slenderman went on to inspire Marble Hornets, a number of video games, and a dedicated wikia site. Stories varied, but a few common themes - that he targeted children; that he would stalk victims and send them mad, that he could teleport, that he had tentacles - soon emerged.

Hollywood, too, took note: Marble Hornets was adapted into a film in 2015 (James Moran's Always Watching: A Marble Hornets Story) and a separate Sony Slenderman project, directed by Sylvain White, is being planned for 2018.

Furthermore, while Slenderman is clearly inspired by older horror imagery (we are pre-programmed to be disturbed by anything-almost human and faceless), his popularity arguably led to a resurgence in tall, thin antagonists, both in horror and in other genres.

The rights to the character itself are protected, but from Pascal Laugier's 2012 The Tall Man, which seemed to directly reference the myth in its marketing (although the film itself does not feature supernatural elements), to Tim Burton's Miss Peregrine's Home for Peculiar Children, to the Sinister series, Slenderman lookalikes were suddenly everywhere.

In the meantime, online fans continued to share stories and build the Slenderman legend. In effect, this was a new, ferociously creative kind of horror storytelling: a sort of democratic, collaborative legend-building, open to anyone who wanted to chip in.

Simultaneously, of course, it was a very old kind of storytelling: modern technology, argued academics such as Andrew Peck, had enabled a "new form of digital folklore". Slenderman was the internet's Big Bad Wolf; its Baba Yaga.

Taylor Brodsky, however, has suggested that the ubiquitous nature of Slenderman's internet presence may have made the fictional monster seem more "authentic" to Geyser, who has been treated for schizophrenia, and Weier.

"This is not a story about the internet preying on children," the director explained last year, ahead of the film's premiere at the SXSW Festival.

"It's about the childish mind falling prey to the highly visceral, and seemingly plausible, universe online we are creating. Slenderman originated from one person's imagination, but has evolved to reflect the rest of us. He is faceless, so can be whomever we most fear. Through scary games, online stories, fan fiction, YouTube serials, Slenderman has become a crowdsourced boogeyman. He is us. And I think the girls recognised something very authentic in that."

Slenderman fans, in contrast, have distanced themselves from the crime and its perpetrators. While the attempted 2014 murder sparked an outpouring of shock and sympathy for the victim on the Slenderman Wikia message boards, users were quick to defend themselves and their community against the resultant backlash.

"...I have already deleted two comments insinuating it is somehow our faults (the entire community) that this crime has taken place," wrote admin Fobaimperis in a post on July 4 2014.

"While I do give my condolensces [SIC] to the families as stated above, I think I can speak for many of us, including the other admins, that we ourselves do not take any responsibility for the crime itself and are in no way morally or legally tied to the situation."

He or she then urged parents to police their children if they had any concerns about their internet usage, and reminded readers that the website had a 13-plus age limit (albeit one that was impossible for site admins to enforce).

"Slender Man has never been an icon for murder or killing, and no major Slender Man series ever depict him as that," the post concluded.

"He is a scary monster, like Freddy Krueger, the aliens from the Alien franchise, a monster from Silent Hill, or even Kayako from The Grudge. That's all."